This research involves studies of the fibrinolytic antiactivator in plasma. This inhibitor of activation of profibrinolysin will be purified and subjected to research on its binding of urokinase and on its properties in general. Especially the blocking of this binding by low molecular weight agents will be checked in this purified system. A search for nontoxic effective blocking agents will be made. The mechanism through which this blocking is achieved will be explored. Effects of oxidizing and reducing agents will be tested.